1. Field of the Invention
The present invention is directed to a copy protection arrangement for video cassette recording and playback. The invention includes both a method of protecting and a protected video casette tape.
2. Description of the Prior Art
Pre-recorded video cassette tapes are now in common use and are becoming increasingly popular with the number of videotape recorders (VTRs) in use. Using two VTRs it is possible to copy a pre-recorded videotape, for example, a commercially produced movie, onto a blank videotape.
Although it is permissible to record and copy "over the air" broadcasted television signals for personal use, it is forbidden by copyright law to copy pre-recorded cassette tapes bearing a copyright notice. However, it is difficult to detect illegal copying done in a private home and on a small scale. Therefore, many pre-recorded cassette manufacturers are hesitant to record certain high value audio-visual works onto video cassettes and then place those cassettes into the public marketplace.
A standard picture frame of a TV receiver adapted for use in the United States or Japan consists of 525 horizontal scan lines which are interlaced every other line to minimize screen flickering. That is, after scanning the odd number lines, e.g. 1, 3, 5, . . . , as shown in FIG. 8 (PRIOR ART), the even numbered lines, e.g. 2, 4, 6, . . . , are scanned as shown by the dotted lines. The method of interlacing a vertical field twice creates a single frame. Scanning by odd number or even number lines only is called "field scanning" and the combination of both is called "frame scanning".
The number of scanning lines varies from country to country, e.g. it is 625 in most countries of Europe, 425 in the United Kingdom and 819 in France. However, all of the major television systems use interlaced scanning. Pictures are transmitted at the rate of 30 frames per second in the United States and Japan and at the rate of 25 frames per second in Europe.
A TV transmission signal includes not only an image signal, but also a blanking signal and a composite synchronizing signal among others. The composite synchronizing signal is simply called a "synchronizing signal". It includes horizontal synchronizing pulses, vertical synchronizing pulses and equalizing pulses. The synchronizing pulses are used for controlling the deflection of the scanning beam used to create passive imagery. That is, the horizontal synchronizing pulse is used for adjusting each horizontal scanning line so that it starts at the left side of the face plate of a Cathode Ray Tube (CRT) and the vertical synchronizing pulse is used for adjusting each field so that it starts at the upper left part of the face plate of the CRT.
FIG. 9A (PRIOR ART) shows the waveform of the synchronizing signals in a first field of interlaced scanning fields. The vertical blanking interval E lies between the end of one field image F and the beginning of the following field image G. In the vertical blanking interval E, there is a composite synchronizing signal including a first equalizing signal partition A having a width of 3H (1H corresponds to 63.5 microseconds), a vertical synchronizing signal partition B of 3H, a second equalizing signal partition C of 3H and a horizontal synchronizing signal partition D of 12H, A, B, C and D following one another.
In FIG. 9A (PRIOR ART), reference letter K indicates the image signal, reference letter L indicates the horizontal blanking interval, reference letter T indicates the horizontal synchronizing pulse, reference letter M indicates the black level, reference letter N indicates the white level, reference letter P indicates the pedestal level, which is the base signal level of the synchronizing signals, and reference letter Q indicates the maximum carrier wave voltage level.
FIG. 9B (PRIOR ART) shows the waveform of the synchronizing signal in the next field of interlaced scanning and is the same as FIG. 9A (PRIOR ART) except for a time lag of 0.5H with respect to FIG. 9A (PRIOR ART).
A VTR may be used to: provide the signal from a pre-recorded video cassette to a TV receiver; or to record onto a blank videotape the signal from a broadcasted TV signal, from a pre-recorded video cassette or from a magnetically encoded master magnetic tape.
A popular type VTR is the helical scanning rotary two-head type shown in FIG. 10 (PRIOR ART). FIG. 11 (PRIOR ART) is a perspective view of the rotary drum 2 having video-heads 1A and 1B, shown in FIG. 10 (PRIOR ART), placed oblique to the tape 3. The upper portion of rotary drum 2 rotates in a direction indicated by arrow 6 and the lower portion 2' of the rotary drum 2 is stationary. Tape 3 is wound to more than half of the circumference of the drum while the drum is rotated at 30 revolutions per minute (rpm) in the direction of the tape flow. The two video heads 1A and 1B alternately run obliquely on the surface of the tape. A video cassette 4 includes a supply reel 41 and a take-up 42. Tape guides 5 are positioned on each side of the rotary drum 2.
When recording a TV signal, as shown in FIG. 12 (PRIOR ART), obliquely running video-tracks 31A and 31B are formed on the surface of tape 3. Each track corresponds to one field. Track 31A is recorded by head 1A shown in FIG. 10 (PRIOR ART) and track 31B is recorded by head 1B shown in FIG. 10 (PRIOR ART). The signal waveform that occurs at the beginning of an interlaced scanning as shown in FIG. 9A (PRIOR ART) and the signal waveform in the field of interlaced scanning, as shown in FIG. 9B (PRIOR ART) are recorded alternately. In FIG. 12 (PRIOR ART), reference numeral 32 indicates the running direction of the tape, reference numeral 33 indicates the running direction of the video head, reference numeral 34 indicates the sonic track, and reference numeral 35 refers to the control track. The vertical synchronizing signal is recorded on the position of the tape indicated by reference numeral 36.
In a videotape recorder, the rotation of the video head and the running of the tape are driven by motors. Unevenness of the rotation of the video head causes a wavering or disorder of the resulting video image. Irregular rotation of the capstan which moves the tape past the head(s) causes picture tremble or slippage of the locus of the play-back video head from the recording track which degrades picture quality. It is necessary to have a servo system to prevent such signal irregularity. The servo system controls the rotation speed and phase of the drive motors by a feedback loop which includes the motors. A typical servo system is shown in FIG. 14 (PRIOR ART).
While the TV signal is recorded, any lead or lag of the rotation phase of the video head 2 is detected by phase comparing, by means of a phase comparator 105, pulses from a pulse generator 104 attached to the rotating video head and a 30 Hz. control pulse 103 generated at a one-half frequency demultiplier 102 (a one-half frequency demultiplier picks up every other pulse). The vertical synchronizing pulse of TV signal 101 is coupled to demultiplier 102. The resulting output control pulse 103 is coupled to one throw of a switch 120. The other throw of switch 120 is coupled to the output of a frequency demultiplier 123, which is driven by a quartz oscillator 122. The pole of switch 120 is coupled to the input of phase comparator 105. The output of pulse generator 104 is also coupled to an input of phase comparator 105. The output of phase comparator 105 is coupled to the inverting input of an amplifier 110, the non-inverting input of which receives an output of a unit 108. The speed of the drum-motor 109 is corrected for head lead or lag by detecting and controlling its speed via unit 108 by utilizing the frequency of a signal 107 from a frequency generator 106 connected to the axis of the rotary video head. A driving amplifier 110 controls the drum motor 2.
The running speed of the tape 3 is monitored by a unit 113 by analyzing the frequency of a signal 112 generated at a frequency generator attached to the axis 111 of the capstan. Any deviation of the running speed of the tape 3 is detected by phase comparator 115, coupled so as to receive signal 112 control pulse 103 and a signal from a frequency demultiplier 124. Any deviation in speed is corrected by controlling the rotating speed of the capstan motor 114. A driving amplifier 116 controls capstan motor 114.
Control pulse 103 is recorded on a control track located at the edge of tape 3 through the control head 117. The control pulse is recorded on control track 35 at the edge of tape 3 as shown in FIG. 12.
A playback servo to reproduce the recorded TV signal is applied by a capstan servo technique method to make up for differences in tracking. While reproducing the recorded TV signal from a VTR cassette, a capstan servo technique method is applied as a playback servo to make up for differences in tracking.
This is a method of controlling the capstan drive which comprises the switching of the switches 120 and 121 to synchronize the phase of the control pulse 103 with the standard signal generated at a frequency demultiplier 123 by converting a constant frequency pulse generated at a quartz oscillator 122 to synchronize the tracking by rotary video head with the recorded track of the running tape. The circuit as shown in FIG. 14 (PRIOR ART) is helpful to reduce the jitter of reproduced video images by keeping the rotation speed of the rotary drum head constant. The circuit can also be used to provide variable speed playback.
One proposal for preventing illegal reproduction of a video cassette is to not record the control signal on the copied tape or to inject pseudo synchronizing signals or to take out a part of the synchronizing signal so as not to reproduce a synchronizing signal that will cooperate with the synchronizing circuitry of a TV receiver.
U.S. Pat. No. 3,963,865 to Songer issued (Jun. 15, 1976) discloses an arrangement for not recording the control signal on the reproduced videotape by using only one and one-half of the vertical synchronizing pulse signals and erasing or otherwise removing the remaining synchronizing pulse signal. The teachings of U.S. Pat. No. 3,963,865 are hereby incorporated by reference as if fully set forth herein. One and one-half of a vertical synchronizing pulse referred to here means one and one-half of the six pulses in the vertical synchronizing signal partition B as shown in FIG. 9A (PRIOR ART). Whether six pulses or one and one-half pulses are used, they are taken out as one square wave pulse in the vertical synchronizing signal separating circuit of the TV receiver. In the Songer patent, the vertical synchronizing pulse is separated from the vertical synchronizing signal circuit by about 48 micro-seconds (3H.times.1.5/6=0.75H).
In the case of videotape recorded by a VTR with helical scanning type rotary two heads described in Tokkaisho 52-130314 (corresponding U.S., if any, unknown at this time), there has been proposed a videotape copy protection method which prevents the reproducing of composite synchronizing signals on a copied videotape. The vertical synchronizing signal is recorded in a "weakened" state on the video track of the videotape. In the weakened state as disclosed in Tokkaisho 52-130314, the base of one square wave pulse width corresponds to about 3H separated by the vertical synchronizing signal circuit and in FIG. 2 of the patent document a suggestion is made to make the width about 96 micro-seconds (1.5H) or to make the signal height to one-half normal value.
All of these methods attempt to make normal playback of a copied video cassette difficult by modifying the vertical synchronizing signal in the copy. The techniques mentioned above were effective in VTRs made prior to 1976 (the year when these patents were applied for) and in specific types of post-1976 VTRs. However, these techniques are no longer effective with the higher technology VTRs now in use. This is primarily because VTRs now in use include automatic waveform regulating functions. As far as the inventor of the present invention has checked, the aforementioned techniques when employed with currently marketed VTRs produced normal pictures from copied video cassette tapes.
In Tokkaisho 54-108612 (corresponding U.S., if any, unknown at this time), it has been proposed to overlap the pulse corresponding to about 100% of the white level to a part of the vertical synchronizing signal to prevent recording by the working of the automatic gain control circuit of the VTR. This method is not always effective. It is more effective on some machines than on others.
None of the known techniques are universally applicable to currently marketed VTRs as an effective countermeasure to prevent illegal copying of videotape cassettes.